Varactor diodes are commonly used as tuning elements in oscillators, as multiplication elements in harmonic generators or multipliers, and as impedance control elements in analog and digital phase shifters at radio microwave or millimeter wave frequencies. Varactor diodes are semiconductor devices also known as variable-capacitance diodes or voltage variable capacitors. In varactor diodes, the capacitance is not constant, but rather varies with the voltage applied to the device. The larger the reverse voltage applied to the diode, the larger the space charge width within the diode and the smaller the capacitance. For example, as impedance control elements in variable phase shifters, the voltage is varied to change the capacitance of the diode and hence the impedance thereby effecting a phase shift change.
A conventional varactor diode incorporates an active semiconductor layer that is sandwiched between a pair of contact layers. The two contact layers are of high conductivity and are of opposite conductivity type from each other, such that one of the contact layers forms a P-N junction with the active layer. The thickness of the active layer, as well as the relative conductivity between the active layer and each of the contact layers, determines the operating characteristics of the varactor.
Varactor diodes are frequently fabricated by initially providing a silicon substrate having a resistivity that matches that of the desired active layer. The heavily doped contact layers are then epitaxially formed and conductor material is deposited on the contact layers. Alternatively, the diode can be formed by starting with a high conductivity substrate and then sequentially forming an epitaxial active layer and epitaxial contact layer.
A common practice utilizes the emitter-base junction of an NPN transistor as a varactor. This practice, however, typically creates a varactor having relatively high leakage current and relatively low capacitance sensitivity. The high base resistance of such a structure also may lead to a low value for Q, or quality factor, which is unfavorable for radio frequency (RF) applications.
Known varactor systems employ a single implant. This is because current double-implant processes do not provide a method for creating varactors having high capacitance density, high capacitance tuning range, low leakage current, and high Q values. The present invention achieves an advance in the art by providing a method and structure for a new varactor design that meets the above challenges.
The problems of the prior art single-implant varactor systems are apparent in the implant profiles readily generated for various implant structures. The deeper the implant, the broader the profile, which means that fewer of the implanted ions are located at their desired depth. On the other hand, shallower implants exhibit much steeper implant profiles. These phenomena are a well-known result of typical ion implantation methods. In a varactor, in order to get the maximum voltage sensitivity in the final device, a steep implant profile is desirable. Thus, to maximize voltage sensitivity, a shallow implant is most preferred. A shallow implant, however, exhibits other undesirable characteristics, namely, high leakage current. The leakage current is determined by the peak doping at the P-N junction. In order to achieve low leakage current, it is desirable to have a low doping concentration on the P-type side of the P-N junction. Together, these two variables—leakage current and voltage sensitivity—are optimized when a steep implant profile having a low peak (i.e., low peak doping concentration) is exhibited. Yet another problem is encountered, however, when the leakage current and voltage sensitivity are optimized in this way. Namely, a low peak doping concentration necessitates use of a limited dose of dopant material. Limited dose of dopant material results in higher series resistance for the device. Series resistance is characterized by the so-called quality factor (Q) of the capacitor. Ideally, a capacitor exhibits zero resistance, only capacitance. In reality, however, some amount of resistance is exhibited that degrades the properties of the capacitor. This relationship between reactance and resistance is represented by Q. As total series resistance approaches a value of zero (i.e., zero resistance, an “ideal” capacitor), Q approaches a value of infinity. Thus, by using a deep implant, low leakage current and low series resistance can be maintained because the implant profile has a low peak doping concentration and very little dopant material remains near the surface. Thus, a varactor formed using a single, deep implant typically will be characterized by high Q and low leakage current, but also an undesirably low voltage sensistivity (i.e., low tuning range). Alternatively, high tuning range may be maintained by utilizing a shallow implant that exhibits a steep implant profile, but high series resistance and/or high leakage current may be experienced. Thus, a varactor formed using a single, shallow implant typically will be characterized by high voltage sensitivity, but low Q and/or high leakage current.